Equilibrium vacancy concentration measurements on aluminum

Abstract

Thermal expansion on high purity aluminum has been measured to study defects in thermal equilibrium by the (ΔL/L 0−Δa/a 0) method. Measurements of the changes in macroscopic length ΔL and lattice parameter Δa were made from room temperature to 637° C. Length changes were measured by a laser interferometer and lattice parameter changes by a X-ray diffractometer technique on the same crystal at identical temperatures. At temperatures above 400° C ΔL/L 0 becomes greater than Δa/a 0, indicating the generation of a noticeable amount of new lattice sites due to vacancy formation. Extrapolation gives a vacancy concentration ΔN/N 0=9.8·10−4 at the melting point (660° C). The experimental findings can be explained by assuming formation of monovacancies and divacancies. Values for the enthalpy and entropy of formation for mono- and divacancies are given.